Bikkurim, Chapter One, Mishnah Three
The Torah never directly states that first fruits are to be brought only from the seven species in which Israel was blessed (Deuteronomy 8:8), wheat, barley, grapes, figs, pomegranates, olive oil and date-honey. Nevertheless, our mishnah teaches that bikkurim are brought only from these species and only from quality types of these species.
1) Bikkurim are brought only from the seven species.
2) Not from dates grown on hills, nor from [the other species] grown in the valley, nor from olives that are not choice.
3) Bikkurim are not to be brought before Shavuot.
4) The people from Mt. Zevoim brought bikkurim prior to Atzeret (Shavuot), but they did not accept from them, on for it is written in the Torah: And the festival of the harvest, the first-fruits of your labors, which you have sown in the field (Exodus 23:16).
Section one: As stated in the introduction, the laws of bikkurim apply only to these species.
Section two: Choice dates grow in the valleys, whereas when it comes to the other species, the choice types grow in the hilly country. Therefore, one can bring dates only from trees that grow in the valleys and from the other species when they grow in the hills. Similarly, one can only bring choice olives.
Section three: The Torah does not actually state when one is to bring bikkurim. However, it alludes to this when it calls Shavuot the festival of the harvest, the first-fruits of your labors. From this verse the rabbis learned that one cannot bring bikkurim before Shavuot, although one can bring them after Shavuot.
Section four: This final section was found also in Hallah 4:10. The people from Mt. Zevoim evidently thought that it was okay to bring first fruits before Shavuot. When they got to the Temple, the priests did not accept them.